Control apparatus of brushless motor controls the rotation of motor while switching the current energizing the stator coil wound in coil shape. The timing for switching the energizing current is determined by detecting the magnetic pole position of the stator. In the conventional control apparatus of brushless motor, the position of rotation of rotor was detected by obtaining the induced voltage generated in the stator coil by the rotation of the rotor and no sensor for position detection is installed separately from considerations of miniaturization of the entire system and so on. More specifically, the voltage at the neutral point of the stator coil and the induced voltage of the stator coil are compared using the comparator, the variation time of output signal of comparator is measured, and the timing for switching the energizing current to the stator coil is determined based on this variation time (for example, refer to Patent Document 1 below).
When the energizing current is switched and current becomes off, the flywheel current flows until the energy accumulated in the coil becomes zero, and the spike induced voltage (spike voltage) that occurs consequently is generated in the stator coil. For this reason, the control apparatus must generate a mask signal, remove such spike voltage as noise, and judge the switching timing. The conventional method to remove spike voltage includes the method of detecting separately the voltage drop between the control apparatus and the stator coil of the brushless motor, generating a reset signal when this voltage drop reaches the same level as the standard voltage, and ignoring the position detection only in the spike voltage space (for example, refer to Patent Document 2 below). To generate a mask signal to mask the spike voltage, means is available to switch on the mask signal when the generation of spike voltage is detected, and when a preset time set beforehand has elapsed the mask signal is switched off (for example, refer to Patent Document 3 below).    [Patent Document 1] Japanese Unexamined Patent Application, First Publication No. S64-8890    [Patent Document 2] Japanese Unexamined Patent Application, First Publication No. H08-182379    [Patent Document 3] Japanese Unexamined Patent Application, First Publication No. H10-28395
The pulse width of the spike voltage in the control method described in Patent Document 1 increases with the increase in the current energizing the stator coil; thus, the point of intersection of the induced voltage and the voltage at the neutral point was sometimes hidden by the spike voltage, depending on the loading condition. In such cases, the position could not be detected, and the brushless motor suffered loss of synchronism.
Moreover, as described in Patent Document 2, to separately detect the voltage drop, a bidirectional diode and bidirectional photocoupler, flip-flop circuit and so on are necessary, so that the problem becomes one of complicated configuration of the apparatus. Furthermore, as described in Patent Document 3, when a mask signal is generated from the pulse of spike voltage, the problem becomes one in which the mask signal is not generated when spike voltage is not generated. In such cases, when spike voltage is generated when the energizing current is switched subsequently, this spike voltage cannot be eliminated, and it becomes difficult to differentiate spike voltage and induced voltage waveforms. Consequently, the problem that occurred was that the position of the rotor cannot be detected, and the timing for switching the current energizing the stator coil cannot be controlled.